1. Technical Field
The present invention is related to a yaw stability control system and method for an automotive vehicle, and specifically to a yaw control system and method capable of correcting for a change in vehicle heading caused by a body-force-disturbance experienced by the vehicle.
2. Background Art
Several vehicle stability control systems, which are used to augment the driving capability of a vehicle operator, currently exist. Those control systems include anti-yaw control-lock system (ABS), traction control system (TCS), and electronic stability augmentation systems. Electronic stability augmentation systems are identified by a number of different names such as electronic stability control (ESC) systems, yaw stability control (YSC) systems. Systems of this kind are also sometimes called ESP (Electronic Stability Program) systems or DSTC (Dynamic Stability Traction Control) systems.
The stability control/augmentation systems are utilized to maintain controlled and stable vehicle operations for improved vehicle and occupant safety. The stability control systems are often used to maintain control of a vehicle following a driver's desired travel direction, to prevent the vehicle from spinning out, and/or plough out, and/or to prevent or mitigate a roll over event.
More specifically, the known yaw stability control systems typically function by comparing the driver's desired direction of vehicle travel based upon the steering wheel angle with the actual path of travel, which is determined from motion sensors located on the vehicle. By regulating the amount of braking at each corner of the vehicle and the traction force of the vehicle and steering, the desired path of travel may be maintained.
Existing stability control systems are designed to correct undesired vehicle motion caused by a tire-force-disturbance, such as a tire-force-differential due to a road surface disturbance or due to a mismatch between the driving intention of a driver and a road surface condition. This mismatch usually happens when there is a significant difference between the front and the rear tire lateral forces applied to the vehicle (referred to as the lateral tire-force-differential), or there is a significant difference between the right and the left tire longitudinal tire-forces (referred to as the longitudinal tire-force-differential), or a combination thereof. Such a tire-force-differential is called a tire-force-disturbance (TFD).
The existing yaw stability control systems are generally effective in controlling the undesired vehicle motions due to the aforementioned tire-force-disturbance. The yaw stability control systems activate yaw controls, reduce engine torque, and/or vary the driving torque at individual wheels or axles so as to generate an active tire-force-differential to counteract the effect of the tire-force-disturbance. That is, the control mechanism and the vehicle disturbance are from the same source: the tire-force variations or the tire-force-differentials.
An undesired yaw motion may also be generated by a yaw-moment-disturbance caused when a vehicle receives a force-disturbance other than a tire-force-disturbance. An example of which is an external force-disturbance that is applied to the vehicle body, which is called a body-force-disturbance (BFD). A body-force-disturbance may occur when a vehicle hits a fixed object, such as a tree, or when the vehicle is hit by another moving object, such as a vehicle. A body-force-disturbance may also occur when the vehicle experiences a sudden strong wind gust applied to the vehicle body.
While the magnitude of the tire-force-disturbance is limited by the driving condition of the road surface, the magnitude of a body-force-disturbance can be much higher. For example, the collision of two moving vehicles may generate a body-force-disturbance with a magnitude that is several factors larger than the total tire-forces. In a light collision, the magnitude of the body-force-disturbance might be very close to the total tire-force. The body-force-disturbance is different from the tire-force-disturbance generated from the tire-force-differentials. A vehicle experiencing a body-force-disturbance may have balanced tire-forces (i.e., there are no significant tire-force-differentials among the 4 tires) and may have larger than normal vehicle motion such as yaw motion.
The yaw stability control systems that exist in today's production vehicles are designed to provide stabilization assistance for the driver during normal road conditions, i.e. when the vehicle's acceleration is limited by the road friction. The efficiency of yaw stability control systems after involved vehicles receive a body-force-disturbance from light collision impacts depends heavily on how the driver reacts after the body-force-disturbance.
It is possible for a first impact to destabilize the vehicle, e.g. by a lateral hit in the rear of the vehicle. In such cases, the vehicle changes travel direction during the short time period of impact due to a resulting large magnitude of vehicle yaw rate, and the driver then has to correct very quickly if he/she should avoid leaving the road, lane, or other intended path of travel. Prior art yaw rate stability controllers usually perform yaw control intervention as a function of yaw rate error, where the yaw rate error is determined as the difference between a yaw rate target determined based on the driver's intended steering and a sensed yaw rate measurement from a yaw rate sensor, and will hence not intervene after the vehicle has stabilized, since there is no large yaw rate error even if there is a large yaw angle (heading angle) error.
In many driving situations, it may be advantageous if the operation range of the stability control is extended to the range of vehicle dynamics which involves the application of a body-force-disturbance, e.g. a light vehicle-to-vehicle impact. Herewith is proposed a way to keep the stability control activated during a body-force-disturbance event such that it is possible to eliminate a vehicle yaw angle resulting from a body-force-disturbance and return the vehicle to a desired heading which may be the pre body-force-disturbance yaw angle, i.e. vehicle heading.